1. Field of the Invention
This invention relates to a method and an apparatus for controlling a stepping motor in an optical storage system, and more particularly, to a method and an apparatus for implementing a short seek through controlling the stepping motor.
2. Description of the Prior Art
In a common optical storage system, when a user accesses data on an optical disk, a pick-up head moves toward the target track on the optical disk to read or write data. Take an optical disk drive for example with the following description of accessing data and a related driving system. Please refer to FIG. 1, which is a schematic diagram of a prior art optical disk drive 10. The optical disk drive 10 comprises a spindle motor 11 fixed in the optical disk drive 10, a sled motor 12, a sliding rail 14, a sled 15, a pick-up head 16 for reading data, and a control module 18 acting as a control system of the optical disk drive 10 for controlling operations of the optical disk drive 10. In addition, there is an optical disk 22 with a plurality of tracks 24 arranged concentrically around the center of optical disk 22 shown in FIG. 1 for storing data. In this case, the main function of the spindle motor 11 is to carry and spin the optical disk 22 for allowing the pick-up head 16 to access data stored on the optical disk 22. In order to make the optical disk drive 10 capable of successfully accessing data, the sliding rail 14 is placed along a radial direction (i.e., the arrowed direction A0 in FIG. 1) of the optical disk 22, and the pick-up head 16 is placed on a sled 15. The sled 15 moves along the sliding rail 14 and is connected to a sled motor 12, which drives the sled 15 by an actuator, such as a set of gear wheels or lead screws. Accordingly, the pick-up head 16 moves to and fro to conduct a long seek along the sliding rail 14.
Please refer to FIG. 2, which is a side elevational diagram of the embodiment shown in FIG. 1. The object lens 26 is placed movably on the pick-up head 16 and is able to conduct a short seek along the radial direction of optical disk 22, whereby the object lens 26 locks precisely onto the track 24 of optical disk 22 for reading data stored on the optical disk 22. Therefore, the moving distance is an absolute distance shifted by the sled 15 plus a distance shifted by the object lens 26 relating to sled 15, when changing to the desired target track. In other words, the object lens 26 has a fast dynamic characteristic, meaning it moves and stops instantly, while the sled 15 has a slow dynamic characteristic, meaning it moves and stops slowly. When the pick-up head 16 reads data sequentially on the optical disk 22, the sled motor 12 drives the sled 15, thereby bringing the pick-up head 16 within the vicinity of the target position of the optical disk 22, and then drives the object lens 26 to lock onto the target track precisely for accessing data. As the target track changes, the object lens 26 moves slightly in the radial direction of the optical disk 22 while at the same time the object lens 26 shifts away from the center of the pick-up head 16, all of which makes the signal quality worse and the servo system of the optical disk drive 10 unstable. As a result, the control module 18 drives the sled motor 12 causing the sled 15 to bring the pick-up head 16, which moves toward the object lens 26, and to ensure that the object lens 26 is at about the center of pick-up head 16 for acquiring better quality of the optical signal.
When the optical disk drive 10 randomly accesses data on the optical disk 22, it is necessary to “jump” between tracks. The track seeking or track jumping performed by the pick-up head 16 is divided into two categories short seek and long seek according to quantity of tracks between a current position of pick-up head 16 and a target track. The short seek has an allowable maximum distance so that any jumping distance that is longer than the allowable maximum distance is conducted by a long-seeking mechanism. Concerning the long seek, the sled 15 accelerates the pick-up head 16 gradually until a predetermined rate is reached and then slows down the pick-up head 16 as it approaches the target track. The benefit of long seek is quick speed, but it has a drawback of locating a track far from the target track after the long seek is completed. Therefore, the track-seeking operation comprises not only long seek but short seek as well, which is more precise. When the pick-up head 16 performs a short seek, the sled motor 12 will not drive the sled 15 or pick-up head 16 too fast in order to avoid being far from the target track suddenly, which makes the short seek more precise. However, as mentioned above, the short seek only covers a short distance and is slower than the long seek.
Please refer to FIG. 1, we can call the sled motor 12, sled 15, pick-up head 16, object lens 26, and control module 18 of the optical disk drive 10 collectively as a sled servo feedback control system. The sled servo feedback control system drives the sled motor 12 to cause the sled 15 to approach the object lens 26 according to the shift distance between the object lens 26 and the center of the pick-up head 16. In other words, sled 15 moves if and only if there is a shift distance present. When the pick-up head 16 (via the object lens 26) accesses data on the optical disk 22 sequentially, the object lens 26 moves slowly through a short distance. Although sled 15 moves slowly, it is able to follow the object lens 26. During the short seek, the object lens 26 moves forward to the target track quickly across a long distance according to a predetermined speed curve. Please refer to FIG. 3, which is a diagram illustrating the speed curve of the object lens 26 and the sled 15 shown in FIG. 2 and a curve of the shift distance between the object lens 26 and the sled 15. At the beginning of a short seek, the object lens 26 shifts to the target track quickly, which increases the shift distance suddenly. When certain of shift distance is achieved, the shift distance is detected by the sled servo feedback control system, which drives the sled motor 12. According to the slow dynamic characteristic of the sled 15, the object lens 26 becomes separated from the center of the pick-up head 16 by a long distance when the sled 15 starts moving. During this time, when the object lens 26 is seriously apart from the center of pick-up head 16, the quality of optical signal becomes worse, and the system falls into an unstable situation. At the end of the short seek, the object lens 26 reaches the target position and stops according to the predetermined speed curve; however, the sled 15 continues to move forward because the shift distance is not equal to zero. The sled servo feedback control system instructs the sled motor 12 to stop until the shift distance becomes zero. However, the sled 15 cannot stop instantly because of its slow dynamic characteristic. We can see clearly in FIG. 3, when the sled 15 is ordered to stop, the sled 15 continues to move, and the shift distance in the opposite direction increases until the sled 15 stops all because the sled 15 cannot stop instantly. Then the sled 15 moves back across the opposite shift distance. In this way, there is a huge shift distance between the object lens 26 and the pick-up head 16, and the sled 15 that moves to and fro may make the system unstable.
In brief, the prior art uses the shift distance between object lens 26 and sled 15 to control the sled motor 12 for driving the sled 15 and the pick-up head 16 in a short seek. This way may lower the quality of the optical signal and make the system unstable because of the slow dynamic characteristic of the sled 15. The above-mentioned problem can be solved by decreasing the allowable maximum distance of a short seek or slowing down the sled 15 for stability, but these solutions also reduce the efficiency of accessing data and degrade the performance of the optical disk drive.